Liquid level protection system for refrigeration compressor

ABSTRACT

A control circuit for protecting a refrigration system against a high liquid level condition includes a thermistor. The thermistor has a negative temperature coefficient of resistivity and is mounted in a housing wall above the normal liquid level, where the liquid includes at least some refrigerant. The thermistor is connected in a circuit which is normally energized, so that the thermistor is self-energized and its impedance is low. If refrigerant accumulates and contacts the thermistor, the refirgerant boils off and rapidly cools the thermistor, increasing its impedance. This increase in impedance provides an increased voltage drop across the thermistor which is used to trigger a semiconductor switch, which in turn interrupts the circuit to the electrical motor.

United stateS Patent 1 1 Norbeck 1 1 3,744,267 1451 July 10,1973

[ LIQUID LEVEL PROTECTION SYSTEM FOR REFRIGERATION COMPRESSOR 3,419,2l4 l2/l968 Evalds 236/78 Primary Exarr zineF-William E. Wayner [75] Inventor: Dean Norbeck Yorkf Attomey- Donald W. Banner, James J. Jennings, Jr., [73] Assignee: Borg-Warner Corporation, Chicago, etal- Ill. [22] Filed: Mar. 15, 1972 [57] ABSTRAfCT A control c1rcu1t for protectmg a refngratlon system pp 234,822 against a high liquid level condition includes a thermistor. The thermistor has a negative temperature coefficient of resistivity and is mounted in a housing wall [52] U S C :4:g?b? a f i above the normal liquid level, where the liquid includes [51] Int. C] Fz'sb 47/00 at least some refrigerant. The thermistor is connected [58] Field 85 192 in a circuit which is normally energized, so that the 62/l93 thermistor is self-energized and its impedance is low. If i 2 refrigerant accumulates and contacts the thermistor, the refrigerant boils off and rapidly cools the thermis- [56] References cued tor, increasing its impedance. This increase in impedance provides an increased voltage drop across the UNITED STATES PATENTS thermistor which is used to trigger a semiconductor 2 K: et 2 3 switch, which in turn interrupts the circuit to the elecul 1 3,411,313 11/1968 Brown et al. 62/192 meal motor 11 Claims, 2 Drawing Figures 11o: Control ler 13 Liquid 41 01122 29 Level C o ntrol Ci rcu it 18 Control 15 Compressor 17' Power f Control Unlt Circuit PATENTEU JUL 1 0 i973 l liy. 12 I 40o-- MotorlOo I -Controller 11c 00 Compressor O 4 41' 'oii'22' l3 Liquid 29 Level i Control CI'FCuil 1 Control 15 Compressor 17 Power Control Unit Circuit l4 16 FIG. 2

T 21 18b IQ 23b Control Circuit 20 LIQUID LEVEL PROTECTION SYSTEM FOR REFRIGERATION COMPRESSOR BACKGROUND OF THE INVENTION In the refrigeration art compressors are generally used to raise the temperature and pressure of the gaseous refrigerant before it enters the condensor to remove heat from the system. A hermetic compressor generally has a motor in the upper portion of the housing, coupled over a shaft to the compressor itself in the lower portion of the housing. Oil can be contained in the bottom of the housing, where it contacts the shaft to provide lubrication for the driving system. However if refrigerant enters the lower part of the housing and increases the liquid level, the liquid can enter the compressor cylinders and damage the compressor. Various systems have been devised to protect against such a high liquid level condition in compressors (or in the receiver or other system component), but none has proved satisfactory.

It is therefore a primary consideration of the present invention to provide a high liquid level protection arrangement for a compressor, which is economical to construct and which rapidly interrupts the energizing circuit for the electric motor when the high liquid level is detected.

Another important consideration of the invention is the provision of such a protective arrangement which is effective when the liquid level is excessive at the time the system is energized to prevent energization of the electric motor.

SUMMARY OF THE INVENTION The present invention includes a control circuit for protecting a refrigeration system which includes an electric motor. The system includes a housing for retaining liquid which has at least some refrigerant at a given level in its lower portion. The control circuit includes a voltage divider circuit, having an impedance component coupled over a common terminal to an element which has a negative temperature coefficient of resistivity; this element is mounted in the housing above the given liquid level. An a-c voltage is applied over a pair of conductors to the voltage divider circuit. A rectifier is coupled to one of the conductors, and a d-c relay has its winding coupled between this rectifier and the other conductor. The relay has a contact set connected to regulate power transfer to the electric motor. A first semiconductor switch having input, output and gate connections is coupled in parallel with the relay winding. A filter capacitor is coupled between the rectifier and the other conductor, to maintain a d-c energizing voltage across the first semiconductor switch. Circuit means, including a second semiconductor switch, is coupled between the gate of the first semiconductor switch and the common terminal in the voltage divider circuit. This provides a tum-on signal for the first semiconductor switch as a function of an impedance change in the element with the negative temperature coefficient of resistivity, caused by contact with the refrigerant-containing liquid above the given level in the housing and cooling of the element as the refrigerant boils off.

THE DRAWING In the several figures of the drawing, like reference numerals identify like components, and in the drawing:

FIG. 1 is a block diagram showing the incorporation of the control circuit of this invention with an energizing system for a motor-driven compressor; and

FIG. 2 is a schematic diagram depicting details of the control circuit shown generally in FIG. 1.

GENERAL SYSTEM DESCRIPTION FIG. 1 depicts a general arrangement for energizing a compressor 10. Three phase a-c energy is received from a conventional power source (not shown) over lines 11a, 1 lb and 1 1c and applied to maintain controller 12. Energy from one phase circuit is passed over line 13 to power control unit 14; of course line 13 represents two conductors but a single line is depicted for simplicity of explanation. This energy is then passed over line 15 to compressor control circuit 16, which also receives a control signal over line 17. This control signal can be a temperature-representative signal from a thermostat. The output signal from control circuit 16 is passed over line 18 to a liquid level control circuit 20.

'In accordance with this invention, a thermistor 21 is positioned in the crankcase of compressor 10 above a reference level 4] representing the normal upper surface of the liquid 22, including oil and refrigerant. Under normal conditions the liquid level remains below the given level 41, when additional refrigerant does not leak down to increase the level of the refrigerant-andoil mixture. The signal from thermistor 21 is applied over line 23 to liquid level control circuit 20. The output signal from circuit 20 is passed over line 29 to controller 12. If the oil-refrigerant liquid rises above level 41, it represents a dangerous and undesired condition, in that liquid could enter the compressor cylinders and damage the compressor. When the liquid level in compressor 10 is below thermistor 21, controller 12 is effective to complete a circuit for passing electrical energy over lines 40a, 40b and 400 to energize motor 10a in the upper portion of the compressor assembly. Thus the liquid level control circuit 20 is not effective to energize and deenergize the compressor itself, but rather is in series with the signal from compressor control circuit 16 to regulate operation of controller 12. With this general view of the invention, a more detailed description of the liquid level control circuit 20 will now be set out in connection with FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION For convenience thermistor element 21 is shown within control circuit 20 in FIG. 2, although physically it is mounted in the crankcase of compressor 10 as shown generally in FIG. 1. Electrically thermistor 21 is coupled over a common terminal 24 to an impedance component 25. Component 25 is shown as a resistor in FIG. 2, and with thermistor 21 forms a voltage divider circuit between a pair of conductors 18a, 18b over which the a-c voltage is applied to control circuit 20. Another series circuit between conductors 18a and 18b includes a thermostat 26, a diode 27, a resistor 28, and a relay winding 42. Relay winding 42 with its associated contact set 43 can be considered as a switching means for regulating de-energization of the compressor motor when the excess liquid level condition is detected.

Connected in parallel with relay winding 42 is a series circuit including a radiation-emitting unit 31 and a first semiconductor switch 32. Unit 31 is shown as a lightemitting diode (LED), because such units provide a visible indication of current flow and exhibit only a very small voltage drop, usually less than 2 volts. Conventional indicator lamps (such as incandescent bulbs) which have a drop of several volts may not be workable in the circuit, ashe units 31, 32 must provide a virtural short circuit across winding 42 when switch 32 is turned on. Switch 32 can be a silicon-controlledrectifier (SCR) or any other suitable switch with input, output and gate connections. Thev input and output connections are coupled in series with unit 31 as shown. The gate connection of switch 32 is coupled over a series circuit including a resistor 33, a second semiconductor switch 34, a diode 35, and a resistor 45 to common terminal 24. The second switch 34 can be another SCR, with its gate coupled over resistor 46 to conductor 18b. A Zener diode 47 is coupled between the anode and the gate of second switch 34 as shown. A resistor 36 is connected as shown between the oath ode of switch 34 and conductor 18b. A capacitor 37 is coupled between resistor 45 and conductor 18b to complete an RC filter 45, 37 for protection against noise and other transients which might otherwise effect spurious triggering of SCR 32. Another filter capacitor 38 is coupled between diode7 and line 18b to filter the d-c voltage provided by diode 27 for energizinghe d-c relay winding 42, and for maintaining first SCR 32 in the conducting state after it is gated on.

In operation it is initially assumed that the oil-andrefrigerant liquid level is below the given level 41, that a-c input voltage is applied over input lines 11al1c, and over conductors 18a and 18b to control circuit 20 shown in FIG. 2. This a-c energy passes over thermostat or switch 26 and is rectified by diode 27 to provide d-c potential for energizing relay winding 42, closing contact set 43. This in turn energizes relay winding 30, closing contact set 44 to complete the energy transfer path between conductors 11a and 40a. Additional contact sets (not shown) similarly complete the circuits between llb and 40b, and 110 and 400. Accordingly the path for energizing the motor and operating the compressor has been completed.

At this time the a-c energy is also applied to the'voltage divider circuit including impedance 25 and thermistor element 21. Thus the thermistor is a self-heated unit; it can be considered a heater, radiating heat to the interior of the compressor crankcase. As the thermistor is heated, its effective resistance value is decreased, so that most of the potential across the voltage divider circuit appears across resistor 25. The potential across thermistor 21 is sufficiently low so that the positive excursions of the a-c voltage passing through diode 35 are not sufficient to pass any pulse through Zener diode 47 to the gate of SCR 34. With SCR 34 off, there is no drive to the gate of SCR 32,'relay windings 42 and 30 remain energized and contact set 44 is closed.

Assuming now that some refrigerant enters the lower portion of the compressor housing to raise the liquid level sufficiently to contact thermistor 21, a good heat sink is provided for the thermistor. That is, the refrigerant boils off and provides this good heat sink connection. This boiling heat transfer is superior to the conductive heat transfer which occurs with a non-boiling liquid contacting the thermistor. An important part of this invention is the appreciation of the rapid cooling provided as the refrigerant boils off the self-heated thermistor, with consequent rapid switching in the liquid level control circuit. With the rapid cooling of the thermistor, its impedance rapidly increases and the voltage drop across the thermistor correspondingly increases. In one embodiment it was found that an increase from about four volts when not contacting the oil-and-refrigerant mixture, to about ten volts when contacting the oil-and-refrigerant liquid, was readily provided, and this voltage increase was sufficient to pass pulses through Zener diode 47 and gate on SCR 34. If the liquid in the compressor crankcase included only oil, a control signal of only a few volts would be realized in this control circuit. However with the mixture of oil and refrigerant, the magnitude of the control signal is of the order of ten volts, and the actuation of the circuit is both positive and very rapid.

Current pulses flow from terminal 24 through resistor 45, diode 35, and Zener diode 47 to the gate of second I nection of SCR 32. Thus SCR 32 is quickly gated on,

effectively shorting relay winding 42 and allowing contact set 43 to open, interrupting the energizing circuit for winding 30 to open the main energizing circuit for the compressor motor 100. With current flow through SCR 32, LED 31 is energized to provide a visible indication of the high liquid level condition. Accordingly the operator recognizes that the compressor has been shut down.

The filter action of capacitor 38 in conjunction with the rectified voltage provided by diode 27 is important. By maintaining an appropriate d-c voltage level, sufficient energizing voltage is provided for the LED-SCR combination so that SCR 32 is maintained in the conducting state after it has been gated on over SCR 34. This is important, as continued conduction of the SCR prevents unwanted energization or chattering of the relay which includes winding 42. The latching action of the relay circuit is also important. By providing holding contact 43 in series with relay winding 30, protection against inadvertent starting of the system is provided. With this latching arrangement the system cannot restart by itself, but the switching means 26 (which may be a thermostat) must be actuated to drop out SCR 32 and prepare the circuit for the next cycle of operation.

The boiling heat transfer, as the refrigerant contacts the thermistor, is very important. This provides a substantial impedance change and consequent large change in the signal voltage developed across the thermistor. This large change in the signal or control voltage is sufficient both to overcome any variations caused by changes in the ambient temperature, and to minimize voltage fluctuations caused by power supply variations.

To re-start the equipment, thermostat switch 26 is momentarily opened to interrupt the current through SCR 32. There can be a separate switch on the thermostat unit in the controlled space for ready access to restart the equipment. On those units where no switch is provided, the dial of the thermostat can be turned to effect the circuit interruption. Use of the switch in conjunction with the thermostat element is preferable because actuation of the switch does not change the room temperature setting on the thermostat. Provided that the oil-and-refrigerant mixture is not contacting thermistor 21, the effective resistance of thermistor 21 will have reached a suitably low value when switch 26 is restored to the closed circuit condition so that there will be no gate drive to the SCR 34, and SCR 32 will be off. Accordingly relay winding 42 will again be energized to close contact set 43, energize winding 30 and complete the power transfer circuit for the compressor. However if thermistor 21 is still being cooled by the boiling heat transfer, SCR 34 will immediately turn on SCR' 32 to prevent the operation of the compressor motor 100. Of course this is also the case if, prior to system start-up, thermistor 21 is contacting the oil-and-refrigerant liquid and is being cooled.

To enable those skilled in the art to make and use the invention with a minimum of experimentation a table of component values and identifications is set out below. With this arrangement and a relatively low'a-c voltage (24 volts) on lines 18a and 18b, the actuation of relay winding 30 was effective to control the three phase, 220 volt energization of the compressor motor. It is understood that these values are given by way of example only and in no sense by way of limitation.

Component Identification or Value 21 025-17358A 32,34 C103YY 87 IN5730B (5.6 volts) 31 ED123 27 OFl94 37 0.56 f, 75 volts 38 l, 50 volts 100 ohms, ll watts 28 2K ohms, 1 watt 33 2K ohms, 5%

36 390 ohms, 5%

45 560 ohms 42 IRA 10016 While only a particular embodiment of the invention has been described and illustrated, it is manifest that various modifications and alterations may be made therein. It is therefore the intention in the appended claims to cover all such modifications and alterations as may fall with the true spirit and scope of the invention.

What is claimed is:

1. A control circuit for protecting a refrigeration system including an electric motor, which system includes a housing for retaining liquid including at least some refrigerant at a given level in the lower portion of the housing, said control circuit comprising:

a voltage divider circuit, including an impedance component coupled over a common terminal to an element having a negative temperature coefficient of resistivity and mounted in said housing above said given level;

a pair of conductors, for applying an a-c voltage across said voltage divider circuit;

a rectifier coupled to one of said conductors;

a d-c relay, having a winding coupled between said rectifier and the other of said conductors, and having a contact set connected to regulate power transfer to the electric motor;

a first semiconductor switch having input, output, and gate connections, coupled in parallel with said relay winding;

a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch; and

circuit means, including a second semiconductor switch, coupled between the gate connection of the first semiconductor switch and the common terminal in the voltage divider circuit, to provide a turnon signal for the first semiconductor switch as a function of an impedance change in said element signifying cooling as the refrigerant liquid contacts the element and boils off.

2. A control circuit as claimed in claim 1, further comprising a radiation emitting unit coupled in series with the input and output connections of the first semiconductor switch, to provide a visible indication of the first semiconductor switch conduction which interrupts the motor circuit.

3. A control circuit as claimed in claim 1, in which said second semiconductor switch has its anode coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode coupled between the gate and anode of the second semiconductor switch, to provide a d-c trigger signal for rapid energization of the second semiconductor switch and thus gate on the first semiconductor switch upon an increase in the effective resistance of said element as refrigerant contacts said element.

4. A control circuit for protecting a refrigeration compressor. driven by an electric motor, the compressor having a housing which retains refrigerant, and also retains some oil at a given level in the lower portion of the compressor housing, comprising:

a voltage divider circuit, including an impedance component coupled over a common terminal to a thermistor having a negative temperature coefficient of resistivity and mounted in the compressor housing above said given level;

a pair of conductors for applying an a-c voltage across said voltage divider circuit;

a rectifier coupled to one of said conductors;

a d-c relay, having a winding coupled between said rectifier and the other of said conductors and having a contact set connected in the power transfer path for the electric motor;

a first semiconductor switch having input, output and gate connection, coupled in parallel with said relay winding to effectively short the relay winding when the semi-conductor switch is conducting;

a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch and the d-c relay winding; and

circuit means, including a second semiconductor switch, coupled between said first semiconductor switch and the common terminal in the voltage divider circuit, to gate on the first semiconductor switch as the impedance of the thermistor increases to signify increased heat conduction by contact with the refrigerant.

5. A control circuit as claimed in claim 4, in which said second semiconductor switch is coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode is coupled between the anode and the gate of the second semiconductor switch, to provide a turn-on signal for the second semiconductor switch responsive to cooling of the thermistor and an increase in the voltage at the common terminal.

6. A control circuit as claimed in claim 4, and in which said first semiconductor switch is a silicon controlled rectifier having anode, cathode and gate connections, with the anode and cathode connections being coupled in a parallel path with said relay winding.

7. A control circuit as claimed in claim 6, and further comprising a light-emitting diode, coupled in series with said silicon controlled rectifier, to provide a visible indication that the silicon controlled rectifier has been gated on 8. A control circuit for protecting a compressor driven by an electric motor, the compressor having a housing for retaining refrigerant material, and for retaining oil at a reference level in the lower portion of the compressor housing, comprising:

a pair of conductors for receiving an a-c voltage;

a voltage divider circuit coupled between said pair of conductors, including a resistor coupled over a common terminal to a thermistor which has a negative temperature coefficient of resistivity, which thermistor is mounted in the compressor housing above said given level;

a series circuit coupled between said pair of conductors, including a rectifier and the winding of a d-c relay, so that a-c voltage supplied over the pair of conductors is rectified by the rectifier to energize the relay winding, which relay has a contact set connected in the power transfer path for the electric motor;

a first silicon controlled rectiifer, having its anode and cathode connections coupled in parallel with the d-c relay winding, and having a gate connection;

a filter capacitor, coupled in parallel with the anodecathode path of the first silicon controlled rectifier, and in parallel with the d-c relay winding, to maintain a d-c energizing voltage across these components; and

a trigger circuit for said first silicon controlled rectifier coupled between its gate connection and the common terminal in the voltage divider circuit, comprising a diode and a second silicon controlled rectifier, to gate on the first silicon controlled rectifier as the impedance of the thermistor rises when refrigerant contacts the thermistor, boils off to cool the thermistor and thus increases its impedance.

9. A control circuit as claimed in claim 8, further comprising a thermostat switch connected in said series circuit, to interrupt the current path for the first silicon controlled rectifier and afford a reset function in the circuit.

10. A control circuit as claimed in claim 8, and further comprising a transient suppression circuit, including a resistor and a capacitor, coupled in series between one of the pair of conductors and the common terminal in the voltage divider circuit.

11. A control circuit as claimed in claim 8, further comprising a Zener diode coupled between the gate and anode of the second silicon controlled rectifier, to provide a turn-on signal for the second silicon controlled rectifier as the thermistor is cooled and provides an increased voltage at the common terminal.

* i It 

1. A control circuit for protecting a refrigeration system including an electric motor, which system includes a housing for retaining liquid including at least some refrigerant at a given level in the lower portion of the housing, said control circuit comprising: a voltage divider circuit, including an impedance component coupled over a common terminal to an element having a negative temperature coefficient of resistivity and mounted in said housing above said given level; a pair of conductors, for applying an a-c voltage across said voltage divider circuit; a rectifier coupled to one of said conductors; a d-c relay, having a winding coupled between said rectifier and the other of said conductors, and having a contact set connected to regulate power transfer to the electric motor; a first semiconductor switch having input, output, and gate connections, coupled in parallel with said relay winding; a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch; and circuit means, including a second semiconductor switch, coupled between the gate connection of the first semiconductor switch and the common terminal in the voltage divider circuit, to provide a turn-on signal for the first semiconductor switch as a function of an impedance change in said element signifying cooling as the refrigerant liquid contacts the element and boils off.
 2. A control circuit as claimed in claim 1, further comprising a radiation emitting unit coupled in series with the input and output connections of the first semiconductor switch, to provide a visible indication of the first semiconductor switch conduction which interrupts the motor circuit.
 3. A control circuit as claimed in claim 1, in which said second semiconductor switch has its anode coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode coupled between the gate and anode of the second semiconductor switch, to provide a d-c trigger signal for rapid energization of the second semiconductor switch and thus gate on the first semiconductor switch upon an increase in the effective resistance of said element as refrigerant contacts said element.
 4. A control circuit for protecting a refrigeration compressor driven by an electric motor, the compressor having a housing which retains refrigerant, and also retains some oil at a given level in the lower portion of the compressor housing, comprising: a voltage divider circuit, including an impedance component coupled over a common terminal to a thermistor having a negative temperature coefficient of resistivity and mounted in the compressor housing above said given level; a pair of conductors for applying an a-c voltage across said voltage divider circuit; a rectifier coupled to one of said conductors; a d-c relay, having a winding coupled between said rectifier and the other of said conductors and having a contact set connected in the power transfer path for the electric motor; a first semiconductor switch having input, output and gate connection, coupled in parallel with said relay winding to effectively short the relay winding when the semi-conductor switch is conducting; a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch and the d-c relay winding; and circuit means, including a second semiconductor switch, coupled between said first semiconductor switch and the common terminal in the voltage divider circuit, to gate on the first semiconductor switch as the impedance of the thermistor increases to signify increased heat conduction by contact with the refrigerant.
 5. A control circuit as claimed in claim 4, in which said second semiconductor switch is coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode is coupled between the anode and the gate of the second semiconductor switch, to provide a turn-on signal for the second semiconductor switch responsive to cooling of the thermistor and an increase in the voltage at the common terminal.
 6. A control circuit as claimed in claim 4, and in which said first semiconductor switch is a silicon controlled rectifier having anode, cathode and gate connections, with the anode and cathode connections being coupled in a parallel path with said relay winding.
 7. A control circuit as claimed in claim 6, and further comprising a light-emitting diode, coupled in series with said silicon controlled rectifier, to provide a visible indication that the silicon controlled rectifier has been gated on.
 8. A control circuit for protecting a compressor driven by an electric motor, the compressor having a housing for retaining refrigerant material, and for retaining oil at a reference level in the lower portion of the compressor housing, comprising: a pair of conductors for receiving an a-c voltage; a voltage divider circuit coupled between said pair of conductors, including a resistor coupled over a common terminal to a thermistor which has a negative temperature coefficient of resistivity, which thermistor is mounted in the compressor housing above said given level; a series circuit coupled between said pair of conductors, including a rectifier and the winding of a d-c relay, so that a-c voltage supplied over the pair of conductors is rectified by the rectifier to energize the relay winding, which relay has a contact set connected in the power transfer path for the electric motor; a first silicon controlled rectiifer, having its anode and cathode connections coupled in parallel with the d-c relay winding, and having a gate connection; a filter capacitor, coupled in parallel with the anode-cathode path of the first silicon controlled rectifier, and in parallel with the d-c relay winding, to maintain a d-c energizing voltage across these components; and a trigger circuit for said first silicon controlled rectifier coupled between its gate connection and the common terminal in the voltage divider circuit, comprising a diode and a second silicon controlled rectifier, to gate on the first silicon controlled rectifier as the impedance of the thermistor rises when refrigerant contacts the thermistor, boils off to cool the thermistor and thus increases its impedance.
 9. A control circuit as claimed in claim 8, further comprising a thermostat switch connected in said series circuit, to interrupt the current path for the first silicon controlled rectifier and afford a reset function in the circuit.
 10. A control circuit as claimed in claim 8, and further comprising a transient suppression circuit, including a resistor and a capacitor, coupled in series between one of the pair of conductors and the common terminal in the voltage divider circuit.
 11. A control circuit as claimed in claim 8, further comprising a Zener diode coupled between the gate and anode of the second silicon controlled rectifier, to provide a turn-on signal for the second silicon controlled rectifier as the thermistor is cooled and provides an increased voltage at the common terminal. 